Convert CONFIG_SYS_PBSIZE to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_PBSIZE

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/boston64r6_defconfig b/configs/boston64r6_defconfig
index 0aabb22..4e5a4ac 100644
--- a/configs/boston64r6_defconfig
+++ b/configs/boston64r6_defconfig
@@ -20,6 +20,7 @@
 CONFIG_FIT_BEST_MATCH=y
 CONFIG_OF_STDOUT_VIA_ALIAS=y
 CONFIG_SYS_PROMPT="boston # "
+CONFIG_SYS_PBSIZE=282
 # CONFIG_CMD_ELF is not set
 CONFIG_CMD_GREPENV=y
 CONFIG_CMD_MEMTEST=y